The present invention relates to a gas component sensor for detecting oxygen, hydrocarbon fuel, combustible composition after combustion of the fuel and so on, and a diagnosis system and a control system using the gas composition sensor, and a diagnosis method and a control method using the gas composition sensor, and more particularly to a gas composition sensor used for diagnosing exhaust gas purifying performance of a catalytic converter for a vehicle engine, for detecting an air-to-fuel ratio of a general combustion machine and for detecting or alarming fuel gas leakage of municipal gas.
In general, in the field of air-to-fuel ratio control for an engine, there are a system in which an air-to-fuel ratio is controlled to be stabilized to a stoichiometric air-to-fuel ratio by combining a three way catalyst and an oxygen sensor, and a system in which an air-to-fuel ratio is controlled so that lean-burn is effected within a zone between the stoichiometric air-to-fuel ratio and a lean limit boundary using an air-to-fuel sensor. Further, it will become compulsory to install a hydrocarbon sensor on an engine mounted on a vehicle in order to diagnosing the catalyst by the on-board sensor, and it will become indispensable in connection with an exhaust gas regulation to develop a technology of controlling an air-to-fuel ratio in an air intake system of an engine by detecting an exhaust-gas-recirculation gas or a purged fuel vapor in the air intake system.
In the past, diagnosis systems of exhaust gas purifying catalyst for an engine were disclosed, for example, in Japanese Patent Application Laid-Open No. 7-34860, Japanese Patent Application Laid-Open No. 3-293544, Japanese Patent Application Laid-Open No. 4-109021, Japanese Patent Application Laid-Open No. 1-101455, Japanese Patent Application Laid-Open No. 4-17141, Japanese Patent Application Laid-Open No. 3-74540, Japanese Patent Application Laid-Open No. 2-207159, Japanese Patent Application Laid-Open No. 2-33408 and Japanese Patent Application Laid-Open No. 2-30915. In the diagnosis systems, oxygen sensors or air-to-fuel ratio sensors are arranged in both upstream and downstream sides of a catalyst converter provided in an exhaust pipe, and purifying performance for combustibles and so on of the catalyst converter is derived from an inter-correlation of transient wave-forms or a concentration ratio of the detected signals in regard to oxygen or combustibles detected by the sensors.
Instead of the oxygen sensor or the combustible gas sensor described above, it is also proposed to employ a sensor for a specified detecting object such as NO.sub.x, HC, CO or the like. However, the proposal does not propose any practical structure nor any practical material for the sensors, but discloses only a proposal in regard to algorithm for diagnosing a catalyst converter. Even the sensor for a specified detecting object needs to be further improved in its performance and in its durability for practical use.
Other than the sensors described above, there are few kinds of proven gas composition sensors which can accurately measure an amount of combustible composition, remaining oxygen and the like in the combustion gas of an engine under a highly corrosive, soiled and high temperature environment. However, one of the few proven gas composition sensors capable of being used under such an environment is a so-called air-to-fuel ratio sensor which is a gas composition sensor formed by combining a zirconia concentration cell and a member limiting gas diffusion.
Mechanism of gas detecting of the air-to-fuel ratio sensor is that amounts of oxygen and combustibles such as HC, CO, H.sub.2 are linearly measured as oxygen ion currents in positive and negative directions respectively by means of oxidizing reaction and gas ion exchange reaction of catalytic function of platinoid metal electrodes. In order to maintain a required measuring accuracy of the air-to-fuel ratio sensor for a long time period, it is necessary to suppress degradation of the catalytic function due to soiling and corrosion of the electrodes, to maintain temperature of the pair of cells stable and to calibrate the output characteristic using a gas having a known concentration such as atmospheric air.
According to a recent research result on diagnosis of degradation of a catalytic converter for an engine mounted on a vehicle, in the conventional diagnosing method in which oxygen sensors are arranged in the upstream side and the downstream side of the catalytic converter and signal waveforms from the oxygen sensors are compared, an amount of all the combustibles such as CO, HC and the like are measured together and a degradation level of the catalytic converter is diagnosed by the purifying function for the total amount of the combustibles. As a result, the degradation level of the catalytic converter is estimated only by the purifying function of CO which is a dominant component of the combustible gas composition. Therefore, it is revealed and becomes a trouble that there are many cases where the catalytic converter is diagnosed as degraded even though the purifying function for HC of main object to be diagnosed is not degraded so low that the catalyst should be exchanged. The conventional HC sensor and the conventional CO sensor cannot cope with the above problem since they are not proven in exhaust gas measurement at the present state.
On the other hand, although it is a proven technology to measure the total amount of oxygen and the combustible gas composition together using the oxygen sensor and the air-to-fuel ratio sensor, it has not proposed to separately measure each amount of components, HC or CO, for the purpose of diagnosing a catalytic converter. Therefore, there is a problem in diagnosis of purifying performance for HC or CO.
Further, similarly to the case of diagnosis of catalytic converter, neither the HC sensor nor the CO sensor can cope with exhaust gas measurement in the present state since it has not been practically used. Although the oxygen sensor and the air-to-fuel ratio sensor have been practically used in measuring a total amount of oxygen and combustibles in a exhaust gas, it has not been disclosed to separately measure a concentration of HC or CO having different molecular weight.
Furthermore, when vapor fuel from a fuel tank is purged in an intake system of a gasoline engine, an amount of fuel injected from an injection nozzle is controlled so as to be corrected by measuring an amount of the vapor fuel. In a diesel engine, in order to optimize trade-off relationship between NO.sub.x and PM (particulate matter), it is required to measure an amount of exhaust gas recirculated to intake air, that is, an exhaust gas recirculation ratio to perform critical control of the exhaust gas recirculation ratio. In order to cope with the control, it is required to measure the concentration of the vapor fuel in the intake air and the amount of the combustible composition in the exhaust gas separately. However, there is not proposed any sensor capable of sufficiently coping with such measurement.
On the other hand, as for hydrocarbon group gas such as municipal gas used as a fuel for a common combustion apparatus other than the engine, an SnO.sub.2 sensor or the like is practically used as a detecting device for detecting leakage of the gas. However, the sensor is not sufficient in selectiveness for application to the specified gas and in stability of sensing sensitivity which should be improved.
It is required for the alarming device of fuel gas leakage that various kinds of combustible compositions of hydrocarbons having comparatively small molecular weights in the fuel gas can be detected at a high sensitivity by separating from mono-oxide carbon by the sensor, the other hydrocarbons of non-fuel group having comparatively large molecular weights can be automatically discriminated, and the leakage can be judged to be continuous but not transient. These characteristics are also problems in the conventional sensor to be improved. Further, it is required for the device for alarming incomplete combustion in the gas combustion apparatus that noxious CO component in the gas combustion apparatus can be detected at a high sensitivity to alarm, and safety measures such as air ventilation can be performed if necessary. Thereby, it is required to develop a highly sensitive detecting sensor.